Sustained and Episodic Gravity-Flow Deposits and Related Bioturbation Patterns in Paleogene Turbidites (Tierra Del Fuego, Argentina)

Careful ichnologic analysis is an important tool for discrimination of sustained and episodic gravity-flow deposits, as shown from two stacked packages of deep-marine Upper Eocene-Lower Oligocene turbidites at eastern Tierra del Fuego. A 63-m-thick, mudstone-sandstone lower Package I consists of an upward-coarsening-thickening succession, followed by a 91-m-thick Package II with a reverse trend and an erosional lower contact, consisting of pebbly coarse- to fine-grained sandstones occasionally capped by mudstone beds. Package I represents episodic flow deposits (classical turbidites of distal lobe facies) arranged in minor (∼ 3-4 m thick) upwardcoarsening- thickening cycles. The trace fossils Phymatoderma Brongniart, Zoophycos Massalongo, Chondrites von Sternberg, Tasselia Heinzelin, and Paradictyodora Olivero, Buatois, and Scasso are dominant at the tops of the minor cycles in the Td-e turbidite divisions. Package II consists of sustained-flow deposits (hyperpycnites in channel fill and aggradational lobes) organized in fining- and thinningupwards cycles of complex beds ∼ 5-6 m thick capped by hemipelagic mudstones. These channelized and lobe deposits of long-duration flows record abundant interstitial fluid-escape structures (convolute bedding) indicative of rapid deposition and, as expected, are not bioturbated. Only the topmost mudstones contain trace fossils such as Phymatoderma (prevalent), Zoophycos, Chondrites, and Tasselia. During the depositional phase of each sustained flow, continuous, high sedimentation rates precluded the colonization of the substrate by organisms. During episodes between successive sustained flows, hemipelagic beds (the topmost mudstones) accumulated slowly, favoring bioturbation of the substrate. These bioturbated beds are key surfaces for sedimentologic analysis, allowing proper identification of the bounding surfaces between successive, complex deposits of sustained flows.